Depending on the supported standard, a digital subscriber line (DSL) system may be denoted as an xDSL system where “x” may indicate any DSL standard. For instance, “x” may stand for “A” in ADSL2 or ADSL2+ systems, “V” in VDSL or VDSL2 systems, or “F” in G.fast systems. When a transceiver is located at an operator end of the DSL system, the transceiver may be referred to as an xTU-O. On the other hand, when a transceiver is located at a remote or user end (e.g., a customer premise equipment, or CPE), the transceiver may be referred to as an xTU-R. For example, if the DSL system is a G.fast system, a transceiver at an operator side may be referred to as a G.fast transceiver unit at an operator side (FTU-O). Similarly, in the G.fast system, a CPE transceiver may be referred to as a FTU at a remote terminal (FTU-R).
Extremely high crosstalk level in a multi-port G.fast system tends to make it very difficult to optimize the power consumption at the FTU-O side (e.g., central office of a service provider). A port with no payload data to send in the downstream direction still needs to transmit crosstalk information to aid other ports in a vectored group, otherwise signal-to-noise ratio (SNR) degradation and consequently error would occur on the transmitting ports. Up to this point little is known about the real channels a G.fast system is to encounter. Different length in the highly coupled region (proximal region or bundle) can make big difference in crosstalk, where the amount of crosstalk spans from being negligible to being very severe.
For loops with medium-to-severe crosstalk, there are techniques that provide some compensation at the FTU-O side. The problem with these techniques is that there is trade-off between complexity and accuracy, and hence some of such techniques cannot cancel the impact of crosstalk completely. Besides, existing techniques tend to deteriorate at high frequency regions (e.g., greater than 80 MHz). Before deployment, it is difficult to predict the effectiveness of these techniques, and therefore a flexible framework to incorporate with residual crosstalk noise is in need.
Moreover, existing techniques about crosstalk reduction only calculate channel capacity and treat the difference of the resulting capacity to the perfect one as performance loss. This, however, may not work in a real system without switching to a new bit-loading table designed for the degraded SNR.
The complexity of these compensation techniques is another concern, and hence a flexible framework that can incorporate future upgrades is desired.